Fiber optic gyroscope sensing coils have an intrinsic problem in that they are sensitive to environmental factors such as temperature and vibration, or in other words, they have relatively poor environmental resistance.
For example, when a fiber optic gyroscope sensing coil is subject to a temperature variation, clockwise and counter-clockwise light beams propagating through the sensing coil may experience the effect of the temperature variation at different timings, which may induce phase shifts other than the Sagnac phase shifts in the light beams. The phase error due to the effect of a temperature variation is induced by the Shupe effect and called “Shupe error.”
In order to reduce the Shupe error, there have been proposed various methods of winding optical fiber onto a spool to form a sensing coil.
Among typical winding methods, dipolar and quadrupole winding methods are well known in the art. FIGS. 12(a) and (b) show the sensing coils formed by these methods, respectively. Both of these methods are categorized in a symmetrical winding method, in which a length of optical fiber is wound onto a coil spool such that each pair of fiber sections of the optical fiber, which are equidistant in opposite directions from the midpoint of the optical fiber, are beside or nearly beside each other on the coil spool.
More particularly, a length of optical fiber is pre-wound onto a first and a second supply spool and the midpoint of the fiber is located. The midpoint of the fiber is positioned onto the central core of the coil spool, and the fiber on the first supply spool is off-wound therefrom and wound onto the coil spool in clockwise direction to form a first (single or double) layer of turns. Then, the first supply spool is swapped with the second supply spool, and the fiber on the second supply spool is off-wound therefrom and wound onto the coil spool in counter-clockwise direction to form a second (single or double) layer of turns on the first layer.
In the bipolar winding method, this process is repeated while the two supply spools are swapped for every layer, so that clockwise and counter-clockwise single layers (shown by white circles and dark circles, respectively, in FIG. 12(a)) are alternately formed one on the other on the coil spool. In the quadrupolar winding method, this process is repeated while the two supply spools are swapped for every two layers, so that clockwise and counter-clockwise double layers (shown by white circles and dark circles, respectively, in FIG. 12(b)) are alternately formed one on the other on the coil spool.
The symmetrical winding method is intended to have sensing coils in which each pair of fiber sections, which are equidistant in opposite directions from the midpoint of the fiber, would experience as similar temperature variations as possible to each other, so that any phase shifts induced by a temperature variation in the clockwise and counter-clockwise light beams would be nearly the same and cancel out each other so as to maintain reciprocity of light propagation.
There is, however, a problem with the coils wound in symmetrical winding configurations. Since each pair of fiber sections which are equidistant in opposite directions from the midpoint of the fiber are placed in different layers from each other, the respective phase shifts induced by a temperature variation in the clockwise and counter-clockwise propagating light beams, respectively, cannot completely cancel out each other unless the temperature variation is uniform among the layers.
In order to address this problem, various coil winding methods have been proposed so far. FIG. 12(c) shows a sensing coil formed by using either of coil winding methods proposed by Patent Publication No. 1 (Japanese patent application publication No. Hei-2-212,712 (1990-212,712)) and Patent Publication No 2 (Japanese patent application publication No. Hei-4-198,903 (1992-198,903)).
In the coil winding method proposed by Patent Publication No. 1, the winding of a length of optical fiber onto a coil spool begins from the midpoint of the fiber and two halves of the fiber are wound in opposite (i.e., clockwise and counter-clockwise) directions in a symmetrical manner, such that (i) clockwise and counter-clockwise layers (shown by white and dark circles, respectively, in FIG. 12(c)) are formed in opposite sides of the midplane of the spool, and (ii) each pair of fiber sections, which are equidistant in opposite directions from the midpoint of the fiber, are placed (i) in corresponding layers which have opposite winding directions and lie at the same layer level and (ii) at positions equidistant from the midplane of the coil spool. The above also applies to the coil winding method proposed by Patent Publication No. 2.
More particularly, a length of optical fiber is pre-wound onto a first and a second supply spool and the midpoint of the fiber is located. The midpoint of the fiber is positioned onto the core of the coil spool, and the fiber on the first and second supply spools is off-wound therefrom and wound onto the coil spool while the spools are swapped with each other, as with the symmetrical winding method.
Patent Publication No. 3 (Japanese patent application publication No. Hei-9-053,945 (1997-053,945)) proposes a coil winding method, in which a length of optical fiber is pre-wound onto a first and a second supply spool and the midpoint of the fiber is located. The method uses a coil spool having a central core with a pair of flanges, one of which has an exit port near the central core. The fiber section that contains the midpoint of the fiber is threaded through the exit port and guided out of the coil spool. The fiber is off-wound from the first and second supply spools and wound onto the coil spool while the supply spools are swapped with each other, as with the symmetrical winding method. After the winding is completed, the opposite ends of the fiber are connected with each other by fusing, and the fiber section threaded through the exit port and guided out of the spool is severed to form a pair of fiber end sections, which are used as the end sections of the winding of the coil.
Patent Publication No. 4 (The publication of Japanese patent No. 3,002,095) and Patent Publication No. 5 (The publication of Japanese patent No. 2,708,370) propose to encapsulate layers of turns of an optical fiber coil within a potting material in order to reduce the effect of vibration. The potting material preferably has a relatively high elastic modulus (i.e., a relatively high stiffness) for this purpose, while a relatively low elastic modulus is desirable in order to avoid thermo-mechanical stresses. Thus, Patent Publication No. 4 proposes the use of potting materials having elastic moduli in a range from 1,000 p.s.i. (7 MPa) to 20,000 p.s.i. (138 MPa). Patent Publication No. 5 proposes the use of silicone potting materials having glass transition temperatures below −55 degrees Celsius, and thus outside the operational temperature range of gyroscope sensing coils, in order to assure that no significant change in elastic modulus of the potting material will occur during normal gyroscope operation, as well as proposes the addition of carbon black as filler material into silicone potting materials in order to increase stiffness of the latter, as “bare” silicone materials typically have elastic moduli below the lower limit of the above mentioned elastic modulus range.